Portable electronic devices having the ability to communicate with external networks have become increasingly popular and relatively inexpensive in recent years. Examples of such devices are mobile telephones, notebook computers, pagers and personal digital assistants. Such devices often have the capability of sending and/or receiving data over a wireless link that enables the user to exchange information with other users and/or networks whilst the user remains relatively free to move around. The data can be voice data, text and numerical data, for example, which have been put into digital format. This allows the user to hold telephone conversations, access the Internet and/or private computer networks for example. Due to their portability, notebook computers are frequently brought to meetings, conferences and hotels for example, where increasingly the opportunity of accessing a local network or the Internet is given to the user. Such a situation generates a “hot-spot” where a number of users that are physically close to one another (typically 5 m to 200 m) require delivery of data across a wireless link. The nature of conferences and meetings means that the users very frequently demand data a substantially the same time.
Frequently, the wireless link is between a base station transceiver and a mobile transceiver, both capable of sending and receiving data via the electromagnetic spectrum. The ultra-high frequency (UHF) part of the electromagnetic spectrum is most frequently used for this kind of data transmission, which has a wavelength in the range of approximately 1 m to 0.1 m (and frequency of 300 MHz to 3000 MHz), although higher bands (microwave and infrared) can be used for example 17 GHz and 60 GHz. The International Telecommunication Union (ITU), which manages the international allocation of radio spectrum, allocated the bands 890-915 MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink (base station to mobile station) for mobile telecommunications networks in Europe. The base station is usually mounted high on a stationary object such as a building where it can broadcast a signal for the surrounding area. The demand by users for smaller portable electronic devices as described above means that the base station usually comprises a much larger transceiver, whereas the mobile transceiver is much smaller.
There are many difficulties associated with successfully transmitting and receiving data to users over a wireless link. One problem is that of “multipath”. Radio waves emitted from a base station and from a mobile terminal are repeatedly reflected and scattered on their way to the mobile terminal or base station. Thus when they reach the destination the waves will interfere either constructively or destructively, resulting in a signal that can be heavily attenuated. When the mobile terminal starts to move, the received signal begins to vary rapidly with time causing an effect known as “fast fading”. If the mobile terminal stops or is positioned in a fade the signal can be of extremely poor quality, known as “slow fading”. This problem can be particularly acute with laptops and notebooks placed randomly by the user. Multipath dominates over a scale of approximately one wavelength to one half a wavelength. A relatively simple solution is to construct a series of antennae, for example two, each spaced more than this distance apart. Thus, if one antenna receives a signal in a fade, there is a good chance that the signal at the other antenna will not be in a fade. The signals from each antenna can be combined to provide a better output signal. Spacing antennae in this manner is known as using “diversity” to improve signal quality. In the example described the base station usually has antennae arranged in this manner to achieve “receive” diversity since it receives the signal from the mobile terminal (also known as “uplink”). If the base station uses physically separate antennae to transmit to a mobile terminal this is known as “transmit diversity”. A useful example of a method for achieving transmit diversity has been devised by S. Alamouti and is discussed in his paper “A Simple Transmit Diversity Technique for Wireless Communications” IEEE Journal on Select Areas in Communications, Vol. 16, No. 8, October 1998.
Achieving receive diversity at the mobile terminal by spatial separation of antennae is difficult as, particularly with mobile telephones, there is not sufficient room to space them by the required range of one wavelength to one half wavelength. Furthermore, this would add cost to the mobile terminal that would not be welcomed by users.
More recently, with the increase in the popularity of mobile electronic communications devices and the demand for higher bandwidths in terms of data transfer, the scarcity of the spectrum has become a problem. In many countries including USA, Japan and the members of the European Community, the UHF part of the spectrum is allocated by governments for data transmission of this nature. That allocated portion is then further divided into smaller portions and distributed amongst telecommunication suppliers, very frequently to the highest bidder. Thus each supplier must try to obtain the greatest efficiency from their part of the spectrum that is possible, and a wide variety of complicated algorithms have been developed to do this. A further problem is that an increasing number of users results in a greater amount of interference that must also be overcome technically. Thus it is apparent that there is a need for increased capacity on wireless communications networks in terms transferring greater amounts of data over the available frequency bandwidth, improving signal quality at the receiver and accommodating a larger number of users at any one time on a network having a wireless link.
In attempting to achieve greater efficiency from the electromagnetic spectrum there are two main parameters available to the designer: frequency and time. Referring to FIG. 1 the spectrum can be divided by frequency so that each user sends and receives data with a given frequency band all of the time. Such schemes are known as Frequency Division Multiple Access (FDMA) and allow multiple users to use the same base station simultaneously. Alternatively each user is allocated a specific time window or “slot” in which to send or receive data over the entire available frequency band, also shown in FIG. 1. This scheme is known as Time Division Multiple Access (TDMA) and also allows multiple users to use the same base station, effectively simultaneously as far as the user can perceive. Another possibility is to permit all users to use the entire available frequency band all of the time. However, the data of each user is multiplied with a spreading code to ensure that each user receives only the data intended for them. This scheme is know as Code Division Multiple Access and is also shown in FIG. 1. The spreading code is designed to provide uniqueness to enable identification of the data by the mobile terminal. One example of the code used in a CDMA scheme is Gold code. The exact code that is used depends upon the intended function. For example Walsh and Gold codes can be used to enable a mobile terminal to locate and synchronise with the correct data, whereas orthogonal variable spreading factor codes are intended to ensure that each user's allocated channel is kept separate and distinct. The code “spreads” the data over larger frequency bandwidth enabling power per unit frequency (W/Hz) to be reduced, achieving the same bandwidth in bits per second whilst lowering interference.
One area where the aforementioned problems have been extensively addressed is in the mobile telecommunications industry. The mobile telecommunications industry started major expansion in the early 1980s, although mobile telecommunications were investigated before that. Generally the development of the system has been in “generations” (G) that can be summarised as follows:                1G networks (e.g. Nordic Mobile Telephony (NMT), Advanced Mobile Phone System (AMPS), TACS) are considered to be the first analogue cellular systems, which started early 1980s.        2G networks (e.g. Global System for Mobile Communications (GSM)), cdmaOne based on the EIA Interim Stand 95 (IS-95), Digital Advanced Mobile Phone System (DAMPS)) are the first digital cellular systems launched early 1990s.        2.5G networks (e.g. General Packet Radio Service (GPRS), cdma2000 based on the EIA Interim Standard 2000 that provides an evolutionary path to 3G) are the enhanced versions of 2G networks with data rates up to about 144 kbit/s.        3G networks (e.g. Universal Mobile Telecommunications Service (UMTS) Frequency Division Duplex (FDD) and Time Division Duplex (TDD), cdma2000 1xEVDO, cdma2000 3×, Time Division Space Code Multiple Access (TD-SCDMA), Association of Radio Industries and Business (Japan) (ARIB) Wideband CDMA (WCDMA), Enhanced Data for Global Evolution (EDGE), International Mobile Telecommunication 2000 (IMT-2000), Digital European Cordless Telecommunications (DECT)) are the latest cellular networks that have data rates 384 kbit/s and more.        4G is predominantly conceptual at the moment. Some basic 4G research is being done, but no frequencies have been allocated. The Fourth Generation could be ready for implementation around 2012.        
For example, UMTS is a third generation (3G) telecommunications system based on wideband CDMA direct sequence (W-CDMA DS). W-CDMA is similar to CDMA except that the data is spread over a larger frequency bandwidth. FIG. 2 shows examples of the types of code in W-CDMA and their function.
The solution reached for dealing with a large number of users wanting to use a comparatively small part of the available spectrum has been to geographically divide a network into cells. In this way, by keeping the emitted power of base stations and mobile terminals low, together with use of coding schemes as mentioned above, it is possible to distribute frequencies amongst cells so that the same combination of frequencies can re-used in the network, providing those cells are sufficiently distant. It also permits the terminals carried by the user to be made smaller. However, the demand for increased data transfer rates and improved signal quality still persist.
Another more recent technique that has achieved an increase in capacity utilises multiple-input multiple-output transmission techniques (MIMO) in which a multi antenna transmitter sends data to a multi-element receiver. The signal at each receive antenna is different due to the effects of mutlipath as described above, from which the original signal can be re-assembled. Coding and sending the signal spaced in time has been found to be particularly beneficial in achieving increased capacity in MIMO channels. Thus space, time and coding are used to enhance system capacity which is known as “space-time coding”. As the name suggests space-time encoding involves splitting the signal and transmitting it over a number of antennae that are spatially separate and by sending the signal from each antenna at a different time. This achieves a double diversity effect. One piece of software that has achieved good results was designed by Bell Labs and is known is Bell Labs Layered Space-Time code (BLAST—see www.bell-labs.com/project/blast) that uses space-time coding to encode data at the transmitter and re-assemble data at the receiver. However, MIMO techniques rely upon there being multi-element antennae at the receiver that is not practical for many applications, such as mobile phones and PDAs where the space is not normally available to accommodate a number of antenna elements the required distance apart.
One solution that has been proposed is to provide the mobile terminal with one active antenna and several tuneable passive antennae that together form an array www.signal.uu.se/(Publications/pdf/c0114.pdf). This system is known as switched parasitic antennae (SPA). Although the results of the computer simulation were encouraging in terms of replicating full MIMO capacity, the applicant believes that the difficulty of implementing SPA in practice, bearing in mind the likely tuning difficulties due to interaction between the antennae, the mobile terminal and the user, will make costs prohibitive. Furthermore, the fact that several antennae are still necessary in SPA inhibits the mobile terminal from being made smaller.